High duty cycle electrical stimulation therapy

ABSTRACT

In some examples, a medical device is configured to deliver high dose electrical stimulation therapy to a patient by at least generating and delivering an electrical stimulation signal having a relatively high duty cycle, and a stimulation intensity less than a perception or paresthesia threshold intensity level for the patient. The pulses of the electrical stimulation signal may each have a relatively low amplitude, but due at least in part to a relatively high number of pulses per unit of time, a dose of the electrical stimulation may be high enough to elicit a therapeutic response from the patient.

This application is a divisional of U.S. patent application Ser. No.16/061,930, filed Jun. 13, 2018, which is a national stage ofInternational Patent Application No. PCT/US2016/066935, filed Dec. 15,2016, which claims the benefit of U.S. Provisional Patent ApplicationNo. 62/269,768, filed Dec. 18, 2015, the entire content of eachapplication is incorporated herein by reference.

BACKGROUND

The disclosure relates to electrical stimulation therapy.

BACKGROUND

Medical devices may be external or implanted, and may be used to deliverelectrical stimulation therapy to patients to various tissue sites totreat a variety of symptoms or conditions such as chronic pain, tremor,Parkinson's disease, epilepsy, urinary or fecal incontinence, sexualdysfunction, obesity, or gastroparesis. A medical device may deliverelectrical stimulation therapy via one or more leads that includeelectrodes located proximate to target locations associated with thebrain, the spinal cord, pelvic nerves, peripheral nerves, or thegastrointestinal tract of a patient. Hence, electrical stimulation maybe used in different therapeutic applications, such as deep brainstimulation (DBS), spinal cord stimulation (SCS), pelvic stimulation,gastric stimulation, or peripheral nerve field stimulation (PNFS).

A clinician may select values for a number of programmable parameters inorder to define the electrical stimulation therapy to be delivered bythe implantable stimulator to a patient. For example, the clinician mayselect one or more electrodes, a polarity of each selected electrode, avoltage or current amplitude, a pulse width, and a pulse frequency asstimulation parameters. A set of parameters, such as a set includingelectrode combination, electrode polarity, amplitude, pulse width andpulse rate, may be referred to as a program in the sense that theydefine the electrical stimulation therapy to be delivered to thepatient.

SUMMARY

This disclosure describes example medical devices, systems, andtechniques for delivering a relatively high dose of electricalstimulation therapy to a patient per unit of time to treat one or morepatient conditions. In some examples, a medical device is configured todeliver the high dose of electrical stimulation therapy by at leastgenerating and delivering an electrical stimulation signal having arelatively high duty cycle, and stimulation intensity less than aperception or paresthesia threshold intensity level of the patient. Theelectrical stimulation therapy may comprise stimulation pulses that mayeach have relatively low amplitude. Due at least in part to a relativelyhigh number of pulses per unit of time (e.g., per second) and theresulting relatively high energy delivery per unit of time, the dose ofthe electrical stimulation delivered to the patient may be high enoughto elicit a therapeutic response from the patient.

In some examples, the electrical stimulation may have a duty cycle in arange of about 5% to about 50% and a frequency in a range about 1 Hertz(Hz) to about 1400 Hz, such as less than about 1000 Hz, and each of thepulses may have a pulse width less than or equal to about 5 milliseconds(ms), such as in a range of about 0.1 ms to about 5 ms, or in a range ofabout 0.1 ms to about 1 ms. In these examples, the frequency and pulsewidth may be selected such that the electrical stimulation may have aduty cycle in a range of about 5% to about 50%. In addition, thefrequency, amplitude, and pulse width may be selected such that thestimulation intensity less than at least one of a perception thresholdintensity level or a paresthesia threshold intensity level of thepatient.

In some examples, the medical device may deliver a recharge signal(e.g., one or more pulses or other waveforms) to the patient afterdelivering the electrical stimulation signal having the relatively highduty cycle. Each electrical stimulation pulse has a first polarity andthe recharge signal has a second polarity that is opposite to the firstpolarity. For example, in some examples, the medical device may deliverone or more recharge pulses to the patient after delivering a pluralityof pulses of the electrical stimulation signal having the relativelyhigh duty cycle. In this case, a plurality of stimulation pulses may bedelivered without delivery of a recharge pulses between the stimulationpulses, but then the plurality of stimulation pulses may be followed byone or more recharge pulses.

In one example, a method includes generating, by a medical device, anelectrical stimulation signal comprising a plurality of pulses andhaving a duty cycle in a range of about 5% to about 50% and a frequencyin a range of about 1 Hertz to about 1400 Hertz, wherein each of thepulses has a pulse width in a range of about 0.1 millisecond to about 5milliseconds, the electrical stimulation signal having a stimulationintensity less than of at least one of a perception threshold or aparesthesia threshold of a patient; and delivering, by the medicaldevice, the electrical stimulation signal to the patient.

In another example, a method comprises generating, by a medical device,a first electrical stimulation signal according to a first therapyprogram, the first electrical stimulation signal comprising a firstplurality of electrical stimulation pulses; generating, by the medicaldevice, a second electrical stimulation signal according to a secondtherapy program, the second electrical stimulation signal comprising asecond plurality of electrical stimulation pulses, wherein each pulse ofthe first and second electrical stimulation signals has a pulse width ina range of about 0.1 millisecond to about 5 milliseconds; delivering, bythe medical device, the first and second electrical stimulation signalsto a patient via respective subsets of electrodes to generate first andsecond stimulation fields; and delivering, by the medical device, arecharge signal following the delivery of at least one pulse of each ofthe first and second electrical stimulation signals. Delivering thefirst and second electrical stimulation signals comprises interleavingdelivery of the first and second electrical stimulation signals todeliver electrical stimulation pulses at a frequency in a range of about1 Hertz to about 1400 Hertz. The first and second stimulation fields,individually and when overlapping, have stimulation intensities lessthan at least one of: a perception threshold or a paresthesia thresholdof the patient.

In another example, a method comprises determining a paresthesia orperception threshold for a patient; determining, for a selectedfrequency, a strength-duration curve based on the paresthesia orperception threshold; and determining, based on the strength-durationcurve, a set of one or more electrical stimulation parameter values forgenerating an electrical stimulation signal having stimulation intensityless than at least one of the perception threshold or the paresthesiathreshold of the patient, and having a duty cycle in a range of about 5%to about 50%, a frequency in a range of about 1 Hertz to about 1400Hertz, and a pulse width in a range of about 0.1 millisecond to about 5milliseconds.

In another example, a system a stimulation generator configured togenerate and deliver electrical stimulation therapy to a patient; and aprocessor configured to control the stimulation generator to generateand deliver an electrical stimulation signal comprising a plurality ofpulses and having a duty cycle in a range of about 5% to about 50% and afrequency in a range of about 1 Hertz to about 1400 Hertz, wherein eachof the pulses has a pulse width in a range of about 0.1 millisecond toabout 5 milliseconds, the electrical stimulation signal having astimulation intensity less than at least one of a perception thresholdor a paresthesia threshold of the patient.

In another example, a system comprises a plurality of electrodes; astimulation generator configured to generate and deliver electricalstimulation therapy to a patient via one or more subset of theelectrodes; and a processor configured to control the stimulationgenerator to generate a first electrical stimulation signal according toa first therapy program, the first electrical stimulation signalcomprising a first plurality of electrical stimulation pulses, generatea second electrical stimulation signal according to a second therapyprogram, the second electrical stimulation signal comprising a secondplurality of electrical stimulation pulses, wherein each pulse of thefirst and second electrical stimulation signals has a pulse width in arange of about 0.1 millisecond to about 5 milliseconds, deliver thefirst and second electrical stimulation signals to a patient viarespective subsets of electrodes to generate first and secondstimulation fields, wherein the processor is configured to control thestimulation generator to deliver the first and second electricalstimulation signals by at least interleaving delivery of the first andsecond electrical stimulation signals to deliver electrical stimulationpulses at a frequency in a range of about 1 Hertz to about 1400 Hertz,and deliver a recharge signal following the delivery of at least onepulse of each of the first and second electrical stimulation signals.The first and second stimulation fields, individually and whenoverlapping, have stimulation intensities less than at least one of: aperception threshold or a paresthesia threshold of the patient.

In another example, a system comprises a processor configured todetermine a paresthesia or perception threshold stimulation intensitylevel of a patient, determine, for a selected frequency, astrength-duration curve based on the paresthesia or perception thresholdstimulation intensity level, and determine, based on thestrength-duration curve, a set of one or more electrical stimulationparameter values for generating an electrical stimulation signal havingstimulation intensity less than at least one of the perception thresholdor the paresthesia threshold of the patient, and having a duty cycle ina range of about 5% to about 50%, a frequency in a range of about 1Hertz to about 1400 Hertz, and a pulse width in a range of about 0.1millisecond to about 5 milliseconds.

In another example, a system includes means for generating an electricalstimulation signal comprising a plurality of pulses and having a dutycycle in a range of about 5% to about 50% and a frequency in a range ofabout 1 Hertz to about 1400 Hertz, wherein each of the pulses has apulse width in a range of about 0.1 millisecond to about 5 milliseconds,the electrical stimulation signal having a stimulation intensity lessthan at least one of a perception threshold or a paresthesia thresholdof a patient; and means for delivering the electrical stimulation signalto the patient.

In another example, a system includes means for generating a firstelectrical stimulation signal according to a first therapy program, thefirst electrical stimulation signal comprising a first plurality ofelectrical stimulation pulses, and a second electrical stimulationsignal according to a second therapy program, the second electricalstimulation signal comprising a second plurality of electricalstimulation pulses, wherein each pulse of the first and secondelectrical stimulation signals has a pulse width in a range of about 0.1millisecond to about 5 milliseconds; means for delivering the first andsecond electrical stimulation signals to a patient via respectivesubsets of electrodes to generate first and second stimulation fields,wherein the means for delivering delivers the first and secondelectrical stimulation signals by at least interleaving delivery of thefirst and second electrical stimulation signals to deliver electricalstimulation pulses at a frequency in a range of about 1 Hertz to about1400 Hertz; and means for delivering a recharge signal following thedelivery of at least one pulse of each of the first and secondelectrical stimulation signals. The first and second stimulation fields,individually and when overlapping, have stimulation intensities lessthan at least one of: a perception threshold or a paresthesia thresholdof the patient.

In another example, a system includes means for determining aparesthesia or perception threshold for a patient; means fordetermining, for a selected frequency, a strength-duration curve basedon the paresthesia or perception threshold; and means for determining,based on the strength-duration curve, a set of one or more electricalstimulation parameter values for generating an electrical stimulationsignal having stimulation intensity less than at least one of theperception threshold or the paresthesia threshold of the patient, andhaving a duty cycle in a range of about 20% to about 50%, a frequency ina range of about 1 Hertz to about 1400 Hertz, and a pulse width in arange of about 0.1 millisecond to about 5 milliseconds.

In another example, a computer-readable storage medium comprisesinstructions that, when executed by a processor, cause the processor to:control a stimulation generator to generate an electrical stimulationsignal comprising a plurality of pulses and having a duty cycle in arange of about 5% to about 50% and a frequency in a range of about 1Hertz to about 1400 Hertz, wherein each of the pulses has a pulse widthin a range of about 0.1 millisecond to about 5 milliseconds, theelectrical stimulation signal having a stimulation intensity less thanat least one of a perception threshold or a paresthesia threshold of apatient; and control the stimulation generator to deliver the electricalstimulation signal to the patient.

In another example, a computer-readable storage medium comprisesinstructions that, when executed by a processor, cause the processor tocontrol a stimulation generator of a medical device to generate a firstelectrical stimulation signal according to a first therapy program, thefirst electrical stimulation signal comprising a first plurality ofelectrical stimulation pulses; control the stimulation generator of themedical device to generate a second electrical stimulation signalaccording to a second therapy program, the second electrical stimulationsignal comprising a second plurality of electrical stimulation pulses,wherein each pulse of the first and second electrical stimulationsignals has a pulse width in a range of about 0.1 millisecond to about 5milliseconds; control the stimulation generator to deliver the first andsecond electrical stimulation signals to a patient via respectivesubsets of electrodes to generate first and second stimulation fields byat least interleaving delivery of the first and second electricalstimulation signals to deliver electrical stimulation pulses at afrequency in a range of about 1 Hertz to about 1400 Hertz; and controlthe stimulation generator to deliver a recharge signal following thedelivery of at least one pulse of each of the first and secondelectrical stimulation signals. The first and second stimulation fields,individually and when overlapping, have stimulation intensities lessthan at least one of: a perception threshold or a paresthesia thresholdof the patient.

In another example, a computer-readable storage medium comprisesinstructions that, when executed by a processor, cause the processor to:determine a paresthesia or perception threshold for a patient;determine, for a selected frequency, a strength-duration curve based onthe paresthesia or perception threshold; and determine, based on thestrength-duration curve, a set of one or more electrical stimulationparameter values for generating an electrical stimulation signal havingstimulation intensity less than at least one of the perception thresholdor the paresthesia threshold of the patient, and having a duty cycle ina range of about 5% to about 50%, a frequency in a range of about 1Hertz to about 1400 Hertz, and a pulse width in a range of about 0.1millisecond to about 5 milliseconds.

In another aspect, the disclosure is directed to a computer-readablestorage medium, which may be an article of manufacture. Thecomputer-readable storage medium includes computer-readable instructionsfor execution by one or more processors. The instructions cause one ormore processors to perform any part of the techniques described herein.The instructions may be, for example, software instructions, such asthose used to define a software or computer program.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages of examples according to this disclosure will be apparentfrom the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an example system thatincludes a medical device programmer and an implantable medical device(IMD) configured to deliver high dose electrical stimulation therapy toa patient.

FIG. 2 is a block diagram of the example IMD of FIG. 1.

FIG. 3 is a block diagram of the example external programmer of FIG. 1.

FIGS. 4 and 5 illustrate an example of high duty cycle electricalstimulation waveforms.

FIG. 6 illustrates an example burst electrical stimulation waveform.

FIG. 7 illustrates an example high frequency electrical stimulationwaveform.

FIG. 8 is a flow diagram of an example method of programming high doseelectrical stimulation therapy having a stimulation intensity level thatis less than a perception or paresthesia threshold intensity level for apatient.

FIG. 9 is a flow diagram of an example method of determining aperception or paresthesia threshold intensity level for a patient.

DETAILED DESCRIPTION

This disclosure describes example medical devices, systems, andtechniques for delivering electrical stimulation therapy to treat one ormore patient conditions, the electrical stimulation therapy providing arelatively high amount of electrical stimulation per unit of time(referred to herein as a “high dose”) and a stimulation intensity lessthan a perception or paresthesia threshold intensity level of thepatient. The dose of electrical stimulation may be a function of afrequency and pulse width of the pulses. The perception thresholdintensity level may be the lowest determined stimulation intensity levelat which a patient perception of the electrical stimulation occurs, andthe paresthesia threshold intensity level may be the lowest determinedstimulation intensity level at which the electrical stimulation causesparesthesia, for example, within a predetermined time range (e.g., 30seconds) of the patient receiving the electrical stimulation.

The high dose of electrical stimulation therapy described hereindelivers a relatively high amount of energy (e.g., electrical charge) totissue of the patient per unit of time (e.g., one second). For example,the high dose of electrical stimulation therapy may have a chargedelivery of about 100 microCoulombs to about 2,000 microCoulombs persecond. The sufficiency of electrical stimulation in producing a desiredtherapeutic effect may be based on the amount of charge delivered to thetissue of the patient per unit of time. In the case of electricalstimulation pulses, the amount of charge delivered to the tissue of thepatient per unit of time may be calculated by multiplying the electricalcurrent delivered during an electrical pulse by the pulse width, whichyields the amount of electrical charge delivered during a single pulse,and multiplying the amount of electrical charge delivered to the patientfor one pulse by the frequency of the electrical stimulation signal.

The high energy dose electrical stimulation described herein may beprovided by an electrical stimulation signal having a relatively highduty cycle. The duty cycle may be, for example, the percentage of activeelectrical stimulation per unit of time (e.g., one second), and may, forexample, be a product of a frequency of the pulses and a pulse width ofthe pulses. Thus, the duty cycle may, in some examples, by defined by aplurality of pulses per unit of time, rather than a single pulse.However, other waveforms may be used in other examples.

In some examples, a medical device is configured to generate anddeliver, via one or more electrodes, an electrical stimulation signalhaving a high duty cycle and a frequency less than or equal to about1400 Hertz (Hz), such as less than or equal to about 1000 Hz. Thefrequency may, for example, be in a range of about 1 Hz to about 1400Hz, such as about 1000 Hz. The pulses may each have a relatively lowamplitude (e.g., about 1 milliamp (mA) to about 25 mA, such as about 1mA to about 5 mA), which can be the same or may vary between the pulses.In some examples, the duty cycle may be greater than 5% such as in arange of about 5% to about 50%, or about 20% to about 50%, or about 10%to about 40%, or about 20% to about 30%. Thus, in some examples, thefrequency and pulse width of the pulses may be selected such that theelectrical stimulation may have a duty cycle in a range of about 5% toabout 50%, where the frequency is selected to be in a range of about 1Hz to about 1400 Hz (e.g., less than or equal to about 1000 Hz) and thepulse width is selected to be in a range of about 0.1 ms to about 5 ms(e.g., about 0.1 ms to about 1 ms). In some examples, the amplitude ofthe pulses may be selected to provide therapeutic efficacy and so thatthe intensity of the delivered electrical stimulation is less than orequal to one or both of a paresthesia threshold or perception thresholdof the patient.

Due at least in part to a relatively high number of pulses per unit oftime and the selected pulse width, the dose (e.g., charge per seconddelivered) of the electrical stimulation signal may be high enough toelicit a therapeutic response from the patient, even though eachindividual pulse may have a relatively low amplitude. The relatively lowamplitude of the pulses may also help keep the stimulation intensitylevel less than a perception or paresthesia threshold intensity levelfor the patient. In some examples, the plurality of pulses may have aduty cycle in a range of about 5% to about 50% and a frequency less thanor equal to about 1000 Hz, and each of the pulses may have a pulse widthin a range of about 0.1 ms to about 5 ms, such as about 0.1 ms to about1 ms, or about 500 μs to about 1 ms. For example, the plurality ofpulses may have a duty cycle in a range of about 5% to about 50% and afrequency less equal to about 1000 Hz, and each of the pulses may have apulse width less than or equal to about 0.5 ms.

In some examples in which the high duty cycle, relatively lowstimulation intensity electrical stimulation is delivered to a tissuesite in a patient proximate to the spinal cord, the electricalstimulation may modulate nerve fibers and produce pain relief viamechanisms that do not rely on the activation of dorsal column fibers.Although the electrical stimulation may or may not also activate dorsalcolumn fibers, the electrical stimulation may not rely on activation ofdorsal column fibers, which may cause paresthesia, to providetherapeutic efficacy for pain or another patient condition. For example,the high duty cycle electrical stimulation may block endogenous actionpotentials in A-beta fibers at their branch points. A-beta fibers may beinvolved in some forms of chronic pain modulation, and the high dutycycle electrical stimulation may prevent A-fiber information fromreaching the dorsal horn. Activation of dorsal column axons may causeparesthesia. Thus, the pain relief from the high duty cycle electricalstimulation described herein using relatively low amplitude pulses maybe substantially paresthesia-free in some examples and with somepatients. The paresthesia free electrical stimulation may be referred toas subliminal stimulation in some examples.

In some cases, the high duty cycle electrical stimulation describedherein may modulate dural fibers, which may also be responsible for someaspects of pain (e.g., back pain) without causing activation of dorsalcolumn fibers.

The mechanisms by which the high duty cycle, relatively low stimulationintensity electrical stimulation described herein may cause pain reliefmay include inhibition of spinal neurons, modulation of the activity ofthe central nervous system (CNS) and/or brainstem, or descendinginhibition (e.g., suppression of pain messages to the brain).

The high duty cycle electrical stimulation techniques described hereinmay activate neurons in a different way than burst electricalstimulation techniques. In contrast to burst electrical stimulationtechniques, the high duty cycle electrical stimulation described hereinmay provide better targeting of target tissue sites. For a givenelectrical stimulation dose (e.g., energy per second), burst electricalstimulation techniques may result in activation of more neural tissue(e.g., a larger volume of tissue) than the electrical stimulationdescribed herein, which provides electrical stimulation with a higherfrequency to achieve a dose sufficient to elicit a therapeutic responsefrom a patient.

For example, the high duty cycle electrical stimulation described hereinmay deliver pulses having higher amplitudes, shorter pulse widths, orboth higher amplitudes and shorter pulse widths than the burstelectrical stimulation techniques. Compared to burst electricalstimulation techniques, the higher duty cycle described herein may allowfor a larger therapeutic window for the amplitude of electricalstimulation (e.g., a range of values of the stimulation signal amplitudethat provides efficacious electrical stimulation therapy), which mayresult in more freedom to titrate the amplitude of the pulses. Thelarger therapeutic window may help a clinician tailor the electricalstimulation to a particular patient to allow for different neuralmechanisms to be activated in order to elicit a therapeutic responsefrom the patient, e.g., while maintaining the intensity of theelectrical stimulation below a threshold stimulation intensity level. Inaddition, the larger therapeutic window for the amplitude may provide aclinician with more freedom to select therapy parameter values thatbalance power efficiency (power consumed by the IMD when generating theelectrical stimulation) with the therapeutic effect.

In some examples, a therapeutic window is defined as the values of anelectrical stimulation parameter between an efficacy threshold, whichmay be the lowest electrical stimulation parameter value (or highest,depending on the parameter) at which efficacious effects of theelectrical stimulation were first observed for a particular patient, andan adverse-effects threshold, which may be the lowest electricalstimulation parameter value (or highest, depending on the parameter) atwhich adverse effects of the electrical stimulation were first observedfor the patient.

The high duty cycle electrical stimulation described herein may alsoprovide better targeting of target tissue sites compared to highfrequency electrical stimulation techniques, in which a plurality ofpulses are delivered at frequencies greater than or equal to 1.5kilohertz (kHz). For a given dose, the high frequency electricalstimulation techniques may result in activation of more neural tissuethan the high duty cycle electrical stimulation described herein, whichprovides electrical stimulation with wider pulse widths, but at lowerfrequencies than the high frequency electrical stimulation techniques toachieve a dose sufficient to elicit a therapeutic response from apatient. Compared to high frequency electrical stimulation techniques,the lower frequency of the high duty cycle electrical stimulationdescribed herein may allow for a larger therapeutic window for theamplitude of electrical stimulation. As discussed above, a largertherapeutic window may help a clinician tailor the electricalstimulation to a particular patient and may provide the clinician withmore freedom to select therapy parameter values that balance powerefficiency with the therapeutic effect.

FIG. 1 is a conceptual diagram illustrating example system 10 thatincludes an implantable medical device (IMD) 14 configured to deliverelectrical stimulation therapy to patient 12. In the example shown inFIG. 1, IMD 14 is configured to deliver SCS therapy. Although thetechniques described in this disclosure are generally applicable to avariety of medical devices including external and implantable medicaldevices (IMDs), application of such techniques to IMDs and, moreparticularly, implantable electrical stimulators (e.g.,neurostimulators) will be described for purposes of illustration. Moreparticularly, the disclosure will refer to an implantable spinal cordstimulation (SCS) system for purposes of illustration, but withoutlimitation as to other types of medical devices or other therapeuticapplications of medical devices.

As shown in FIG. 1, system 10 includes an IMD 14, leads 16A, 16B, andexternal programmer 18 shown in conjunction with a patient 12, who isordinarily a human patient. In the example of FIG. 1, IMD 14 is animplantable electrical stimulator that is configured to generate anddeliver electrical stimulation therapy to patient 12 via electrodes ofleads 16A, 16B, e.g., for relief of chronic pain or other symptoms. IMD14 may be a chronic electrical stimulator that remains implanted withinpatient 12 for weeks, months, or even years. In other examples, IMD 14may be a temporary, or trial, stimulator used to screen or evaluate theefficacy of electrical stimulation for chronic therapy.

IMD 14 may be constructed of any polymer, metal, or composite materialsufficient to house the components of IMD 14 (e.g., componentsillustrated in FIG. 2) within patient 12. In this example, IMD 14 may beconstructed with a biocompatible housing, such as titanium or stainlesssteel, or a polymeric material such as silicone, polyurethane, or aliquid crystal polymer, and surgically implanted at a site in patient 12near the pelvis, abdomen, or buttocks. In other examples, IMD 14 may beimplanted within other suitable sites within patient 12, which maydepend, for example, on the target site within patient 12 for thedelivery of electrical stimulation therapy. The outer housing of IMD 14may be configured to provide a hermetic seal for components, such as arechargeable power source. In addition, in some examples, the outerhousing of IMD 14 may be selected of a material that facilitatesreceiving energy to charge the rechargeable power source.

Electrical stimulation energy, which may be constant current or constantvoltage based pulses, for example, is delivered from IMD 14 to one ormore target tissue sites of patient 12 via one or more electrodes (notshown) of implantable leads 16A and 16B (collectively “leads 16”). Inthe example of FIG. 1, leads 16 carry electrodes that are placedadjacent to the target tissue of spinal cord 20. One or more of theelectrodes may be disposed at a distal tip of a lead 16 and/or at otherpositions at intermediate points along the lead. Leads 16 may beimplanted and coupled to IMD 14. The electrodes may transfer electricalstimulation generated by an electrical stimulation generator in IMD 14to tissue of patient 12. Although leads 16 may each be a single lead,each lead 16 may include a lead extension or other segments that may aidin implantation or positioning of the respective lead 16. In some otherexamples, IMD 14 may be a leadless stimulator with one or more arrays ofelectrodes arranged on a housing of the stimulator rather than leadsthat extend from the housing. In addition, in some other examples,system 10 may include one lead or more than two leads, each coupled toIMD 14 and directed to similar or different target tissue sites.

The electrodes of leads 16 may be electrode pads on a paddle lead,circular (e.g., ring) electrodes surrounding the body of the lead,conformable electrodes, cuff electrodes, segmented electrodes (e.g.,electrodes disposed at different circumferential positions around thelead instead of a continuous ring electrode), or any other type ofelectrodes capable of forming unipolar, bipolar or multipolar electrodecombinations for therapy. Ring electrodes arranged at different axialpositions at the distal ends of lead 16 will be described for purposesof illustration.

The deployment of electrodes via leads 16 is described for purposes ofillustration, but arrays of electrodes may be deployed in differentways. For example, a housing associated with a leadless stimulator maycarry arrays of electrodes, e.g., rows and/or columns (or otherpatterns), to which shifting operations may be applied. Such electrodesmay be arranged as surface electrodes, ring electrodes, or protrusions.As a further alternative, electrode arrays may be formed by rows and/orcolumns of electrodes on one or more paddle leads. In some examples,electrode arrays may include electrode segments, which may be arrangedat respective positions around a periphery of a lead, e.g., arranged inthe form of one or more segmented rings around a circumference of acylindrical lead.

The therapy parameters for a therapy program (also referred to herein asa set of electrical stimulation parameter values) that controls deliveryof stimulation therapy by IMD 14 through the electrodes of leads 16 mayinclude information identifying which electrodes have been selected fordelivery of stimulation according to the therapy program, the polaritiesof the selected electrodes, i.e., the electrode configuration for theprogram, and voltage or current amplitude, pulse rate, and pulse widthof stimulation delivered by the electrodes. Delivery of stimulationpulses will be described for purposes of illustration. However,electrical stimulation may be delivered in other forms such ascontinuous waveforms. Programs that control delivery of other therapiesby IMD 14 may include other parameters, e.g., such as rate or the likein the case IMD 14 is also configured for drug delivery.

Although FIG. 1 is directed to SCS therapy, e.g., used to treat pain, inother examples system 10 may be configured to treat any other conditionthat may benefit from electrical stimulation therapy. For example,system 10 may be used to treat tremor, Parkinson's disease, epilepsy, apelvic floor disorder (e.g., urinary incontinence or other bladderdysfunction, fecal incontinence, pelvic pain, bowel dysfunction, orsexual dysfunction), obesity, gastroparesis, or psychiatric disorders(e.g., depression, mania, obsessive compulsive disorder, anxietydisorders, and the like). In this manner, system 10 may be configured toprovide therapy taking the form of deep brain stimulation (DBS),peripheral nerve stimulation (PNS), peripheral nerve field stimulation(PNFS), cortical stimulation (CS), pelvic floor stimulation,gastrointestinal stimulation, or any other stimulation therapy capableof treating a condition of patient 12.

In some examples, lead 16 may include one or more sensors configured toallow IMD 14 to monitor one or more parameters of patient 12. The one ormore sensors may be provided in addition to, or in place of, therapydelivery by lead 16.

IMD 14 is configured to deliver high dose electrical stimulation therapyto patient 12 via selected combinations of electrodes carried by one orboth of leads 16, alone or in combination with an electrode carried byor defined by an outer housing of IMD 14. The target tissue for the highdose electrical stimulation therapy may be any tissue affected byelectrical stimulation, which may be in the form of electricalstimulation pulses or continuous waveforms. In some examples, the targettissue includes nerves, smooth muscle or skeletal muscle. In the exampleillustrated by FIG. 1, the target tissue is tissue proximate spinal cord20, such as within an intrathecal space or epidural space of spinal cord20, or, in some examples, adjacent nerves that branch off of spinal cord20. Leads 16 may be introduced into spinal cord 18 in via any suitableregion, such as the thoracic, cervical or lumbar regions. Stimulation ofspinal cord 18 may, for example, prevent pain signals from travelingthrough spinal cord 20 and to the brain of patient 12. Patient 12 mayperceive the interruption of pain signals as a reduction in pain and,therefore, efficacious therapy results.

IMD 14 generates and delivers electrical stimulation therapy to a targetstimulation site within patient 12 via the electrodes of leads 16 topatient 12 according to one or more therapy programs. A therapy programdefines values for one or more parameters that define an aspect of thetherapy delivered by IMD 14 according to that program. For example, atherapy program that controls delivery of stimulation by IMD 14 in theform of pulses may define values for voltage or current pulse amplitude,pulse width, and pulse rate for stimulation pulses delivered by IMD 14according to that program.

Moreover, in some examples, IMD 14 delivers electrical stimulationtherapy to patient 12 according to multiple therapy programs, which maybe stored as a therapy program group. For example, as described below,in some examples, IMD 14 may deliver different pulses of a high dutycycle electrical stimulation signal via respective electrodecombinations, and each of the electrode combinations may be associatedwith a respective therapy program. The therapy programs may be stored asa group, such that when IMD 14 generates and delivers electricalstimulation therapy via a selected group, IMD 14 delivers high dutycycle electrical stimulation signal via two or more therapy programs.

IMD 14 is configured to deliver a recharge signal (e.g., one or morerecharge pulses or other waveforms), which may help balance a chargeaccumulation that may occur within tissue proximate the electrodes usedto deliver the electrical stimulation. The recharge signal may also bereferred to as a “recovery signal” or a “charge balancing signal” andmay have a polarity opposite to that of the electrical stimulationsignal generated and delivered by IMD 14. While recharge pulses areprimarily referred to herein, in other examples, a recharge signal canhave any suitable waveform.

In some examples, IMD 14 may deliver a recharge signal after delivery ofmultiple pulses of a high duty electrical stimulation signal, which maybe defined by one therapy program or by multiple therapy programs. Thus,rather than charge balancing on a pulse-by-pulse basis (e.g., deliveringone recharge pulse after each electrical stimulation pulse), in someexamples, IMD 14 delivers one or more recharge pulses after delivery oftwo or more electrical stimulation pulses. In some examples, IMD 14delivers a high duty electrical stimulation signal to patient 12according to multiple therapy programs by at least interleaving pulsesof two or more therapy programs, the pulses having a first polarity. Insome of these examples, IMD 14 may wait to deliver one or more rechargepulses until after one or more pulses of each of the therapy programsare delivered, each recharge pulse having a second polarity opposite tothe first polarity. Thus, in some examples, IMD 14 may not deliver anyrecharge signals between therapy programs, but, rather, may withhold thedelivery of one or more recharge signals until after IMD 14 delivers aplurality of pulses according to two or more therapy programs.

In some examples, IMD 14 is configured to generate and deliver high dutycycle electrical stimulation therapy to patient 12 via two or moreelectrodes, e.g., of leads 16 and/or a housing of IMD 14. In someexamples, the high duty cycle electrical stimulation signal may have aduty cycle in a range of about 5% to about 50%, a frequency in a rangeof about 1 Hz to about 1400 Hz (e.g., less than about 1000 Hz in someexamples), and a pulse width less than or equal to about 5 ms, such asabout 0.1 ms to about 5 ms, or about 0.1 ms to about 1 ms. The amplitudeand pulse width of the electrical stimulation signal are selected suchthat a stimulation intensity level of the electrical stimulation signalis less than a perception or paresthesia threshold intensity level forpatient 12. For example, the amplitude may be selected to be in a rangeof about 1 mA to about 25 mA, such as in a range of about 1 mA to about5 mA.

In some examples, IMD 14 delivers the pulses of the high duty cycleelectrical stimulation signal via different electrode combinations. Forexample, IMD 14 may alternate delivery of pulses between two differentelectrode combinations, or may otherwise interleave the pulses using twoor more electrode combinations in any suitable order. Regardless of thenumber of electrode combinations with which IMD 14 delivers the pulses,however, the combination of pulses delivered over time define anelectrical stimulation signal that may have a duty cycle in a range ofabout 5% to about 50% and a frequency in a range of about 1 Hz to about1400 Hz.

A user, such as a clinician or patient 12, may interact with a userinterface of an external programmer 18 to program IMD 14. Programming ofIMD 14 may refer generally to the generation and transfer of commands,programs, or other information to control the operation of IMD 14. Inthis manner, IMD 14 may receive the transferred commands and programsfrom programmer 18 to control stimulation therapy. For example, externalprogrammer 18 may transmit therapy programs, stimulation parameteradjustments, therapy program selections, therapy program groupselections, user input, or other information to control the operation ofIMD 14, e.g., by wireless telemetry or wired connection.

In some cases, external programmer 18 may be characterized as aphysician or clinician programmer if it is primarily intended for use bya physician or clinician. In other cases, external programmer 18 may becharacterized as a patient programmer if it is primarily intended foruse by a patient. A patient programmer may be generally accessible topatient 12 and, in many cases, may be a portable device that mayaccompany patient 12 throughout the patient's daily routine. Forexample, a patient programmer may receive input from patient 12 when thepatient wishes to terminate or change stimulation therapy. In general, aphysician or clinician programmer may support selection and generationof programs by a clinician for use by IMD 14, whereas a patientprogrammer may support adjustment and selection of such programs by apatient during ordinary use. In other examples, external programmer 18may be included, or part of, an external charging device that rechargesa power source of IMD 14. In this manner, a user may program and chargeIMD 14 using one device, or multiple devices.

As described herein, information may be transmitted between externalprogrammer 18 and IMD 14. Therefore, IMD 14 and programmer 18 maycommunicate via wireless communication using any techniques known in theart. Examples of communication techniques may include, for example,radiofrequency (RF) telemetry and inductive coupling, but othertechniques are also contemplated. In some examples, programmer 18 mayinclude a communication head that may be placed proximate to thepatient's body near the IMD 14 implant site in order to improve thequality or security of communication between IMD 14 and programmer 18.Communication between programmer 18 and IMD 14 may occur during powertransmission or separate from power transmission.

Although IMD 14 is generally described herein, techniques of thisdisclosure may also be applicable to external or partially externalmedical device in other examples. For example, IMD 14 may instead beconfigured as an external medical device coupled to one or morepercutaneous medical leads. The external medical device may be achronic, temporary, or trial electrical stimulator. In addition, anexternal electrical stimulator may be used in addition to one or moreIMDs 14 to deliver electrical stimulation described herein.

FIG. 2 is a functional block diagram illustrating various components ofan example IMD 14. In the example shown in FIG. 2, IMD 14 includesprocessor 30, memory 32, stimulation generator 34, telemetry module 36,and power source 38. In other examples, IMD 14 may include a greater orfewer number of components. For example, IMD 14 may also include any oneor more of a sensing module configured to sense one or more patientparameters, an inductive coil to receive power from an external chargingdevice, and a recharge module that manages recharging of power source38.

Processor 30 is operably connected to and configured to accessinformation from memory 32 and to control stimulation generator 34 andtelemetry circuit 36. Components described as processor 30 and otherprocessors within IMD 14, external programmer 20 or any other devicedescribed in this disclosure may each comprise one or more processors,such as one or more microprocessors, digital signal processors (DSPs),application specific integrated circuits (ASICs), field programmablegate arrays (FPGAs), programmable logic circuitry, or the like, eitheralone or in any suitable combination. In general, IMD 14 may compriseany suitable arrangement of hardware (e.g., circuitry), alone or incombination with software and/or firmware, to perform the varioustechniques described herein attributed to IMD 14 and processor 30. Invarious examples, IMD 14 may include one or more processors 30, such asone or more DSPs, ASICs, FPGAs, programmable logic circuitry, or thelike, either alone or in any suitable combination.

Memory 32 may store therapy programs 40 (or other instructions thatspecify therapy parameter values for the therapy provided by stimulationgenerator 34 and IMD 14), operating instructions 42 for execution byprocessor 30, and any other information regarding therapy of patient 12.In some examples, memory 32 may also store instructions forcommunication between IMD 14 and programmer 18, or any otherinstructions required to perform tasks attributed to IMD 14. Memory 32may include separate memories for storing therapy programs, operatinginstructions, and any other data that may benefit from separate physicalmemory modules.

Memory 32 may comprise any suitable, such as random access memory (RAM),read only memory (ROM), programmable read only memory (PROM), erasableprogrammable read only memory (EPROM), electronically erasableprogrammable read only memory (EEPROM), flash memory, comprisingexecutable instructions for causing the one or more processors toperform the actions attributed to them. Although processor 30, therapymodule 34, and telemetry module 36 are described as separate modules, insome examples, processor 30, therapy module 34, and telemetry module 36may be functionally integrated. In some examples, processor 30, therapymodule 34, and telemetry module 36 may correspond to individual hardwareunits, such as ASICs, DSPs, FPGAs, or other hardware units.

Stimulation generator 34 forms a therapy delivery module of IMD 14.Processor 30 controls stimulation generator 34 to generate and deliverelectrical stimulation via electrode combinations formed by a selectedsubset of electrodes 24A-24D, 26A-26D (collectively, “electrodes 24,26”) of leads 16. Stimulation generator 34 may deliver electricalstimulation therapy via electrodes on one or more of leads 16, e.g., asstimulation pulses. Stimulation generator 34 may include stimulationgeneration circuitry to generate stimulation pulses and, in someexamples, switching circuitry to switch the stimulation across differentelectrode combinations, e.g., in response to control by processor 30. Inother examples, stimulation generator 34 may include multiple currentsources to drive more than one electrode combination at one time.

In some examples, processor 30 controls stimulation generator 34 byaccessing memory 32 to selectively access and load at least one of thetherapy programs 40 to stimulation generator 34. The stimulationparameter values of the stored therapy programs 40 may include, forexample, a voltage amplitude, a current amplitude, a pulse frequency, apulse width, a duty cycle, and a subset of electrodes 24, 26 of leads 16for delivering the electrical stimulation signal. An electrodeconfiguration may include the one or more electrodes 24, 26 with whichstimulation generator 34 delivers the electrical stimulation to tissueof a patient, and the associated electrode polarities.

In some examples, IMD 14 may deliver a high duty cycle electricalstimulation signal to a target tissue site within patient 12 via oneelectrode combination, such that all pulses are delivered via the sameelectrode combination. In other examples, IMD 14 may deliver a high dutycycle electrical stimulation signal to a target tissue site withinpatient 12 via two or more electrode combinations, such that IMD 14delivers at least two different pulses of a high duty cycle electricalstimulation signal via respective electrode combinations. The deliveryof different pulses via respective electrode combinations may helptarget the electrical stimulation to a target tissue site (e.g., in thecase of pain relief, the target may be towards a midline of spinal cord20, for example, near the T9-T10 vertebrae). The electrical stimulationdelivered by each electrode combination, which may be referred to as asub-signal, may be interleaved (e.g., delivered at different times) todefine the high duty cycle electrical stimulation signal. In some ofthese examples, each sub-signal is associated with a respective therapyprogram. Thus, processor 30 may control stimulation generator 34 togenerate and deliver a high duty cycle electrical stimulation signal byat least accessing memory 32 to selectively access and load multipletherapy programs 40 to stimulation generator 34.

IMD 14 also includes components to receive power from programmer 18 or aseparate charging device to recharge a battery of power source 38. Powersource 38 may include one or more capacitors, batteries, or other energystorage devices. IMD 14 may thus also include an inductive coil and arecharge module (both not shown) configured to manage the rechargingsession for power source 38. Although inductive coupling may be used torecharge power source 38, other wireless energy transfer techniques mayalternatively be used. Alternatively, power source 38 may not berechargeable.

Processor 30 may also control the exchange of information withprogrammer 18 and/or an external programmer using telemetry module 36.Telemetry module 36 may be configured for wireless communication usingRF protocols, inductive communication protocols, or any other suitabletechnique. To support the wireless communication, telemetry circuit 36may include appropriate electronic components, such as amplifiers,filters, mixers, encoders, decoders, and the like. Processor 30 maytransmit operational information and receive therapy programs or therapyparameter adjustments via telemetry module 36. Also, in some examples,IMD 14 may communicate with other implanted devices, such asstimulators, control devices, or sensors, via telemetry module 36.

FIG. 3 is a block diagram of an example external programmer 18. Whileprogrammer 18 may generally be described as a hand-held device,programmer 18 may be a larger portable device or a more stationarydevice in some examples. In addition, in other examples, programmer 18may be included as part of an external charging device or include thefunctionality of an external charging device. As illustrated in FIG. 3,programmer 18 may include a processor 50, memory 52, user interface 54,telemetry module 56, and power source 58. Memory 52 may storeinstructions that, when executed by processor 50, cause processor 50 andexternal programmer 18 to provide the functionality ascribed to externalprogrammer 18 throughout this disclosure.

Programmer 18 comprises any suitable arrangement of hardware, alone orin combination with software and/or firmware, to perform the techniquesattributed to programmer 18, and processor 50, user interface 54, andtelemetry module 56 of programmer 18. In various examples, processor 50may include one or more processors, such as one or more microprocessors,DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logiccircuitry, as well as any combinations of such components. Programmer 18also, in various examples, may include a memory 52, such as RAM, ROM,PROM, EPROM, EEPROM, flash memory, a hard disk, a CD-ROM, comprisingexecutable instructions for causing the one or more processors toperform the actions attributed to them. Moreover, although processor 50and telemetry module 56 are described as separate modules, in someexamples, processor 50 and telemetry module 56 are functionallyintegrated. In some examples, processor 50 and telemetry module 56correspond to individual hardware units, such as ASICs, DSPs, FPGAs, orother hardware units.

Memory 52 may store instructions that, when executed by processor 50,cause processor 50 and programmer 18 to provide the functionalityascribed to programmer 18 throughout this disclosure. In addition, insome examples, memory 52 stores one or more therapy programs forexecution by IMD 14 to deliver high dose electrical stimulation therapy.

User interface 54 may include a button or keypad, lights, a speaker forvoice commands, a display, such as a liquid crystal (LCD),light-emitting diode (LED), or organic light-emitting diode (OLED). Insome examples the display may be a touch screen. User interface 54 maybe configured to display any information related to the delivery ofstimulation therapy, such as currently selected parameter values,intensity thresholds, or any other therapy information. User interface54 may also receive user input via user interface 54. The input may be,for example, in the form of pressing a button on a keypad or selectingan icon from a touch screen. The input may, for example, requeststarting or stopping electrical stimulation, or requesting some otherchange to the delivery of electrical stimulation.

Telemetry module 56 may support wireless communication between IMD 14and programmer 18 under the control of processor 50. Telemetry module 56may also be configured to communicate with another computing device viawireless communication techniques, or direct communication through awired connection. In some examples, telemetry module 56 may besubstantially similar to telemetry module 36 of IMD 14 described herein,providing wireless communication via an RF or proximal inductive medium.In some examples, telemetry module 56 may include an antenna, which maytake on a variety of forms, such as an internal or external antenna.

Examples of local wireless communication techniques that may be employedto facilitate communication between programmer 18 and IMD 14 include RFcommunication according to the 802.11 or Bluetooth specification sets orother standard or proprietary telemetry protocols. In this manner, otherexternal devices may be capable of communicating with programmer 18without needing to establish a secure wireless connection.

FIG. 4 is a timing diagram of an example high density electricalstimulation signal 60 that IMD 14 may generate and deliver to patient12. Electrical stimulation signal 60 includes a plurality of pulses62A-62G (collectively, “pulses 62”). Although seven pulses are shown inFIG. 4, stimulation signal 60 may include any number of pulses, whichmay depend on the time period over which IMD 14 delivers stimulationsignal 60 to patient 12. Each pulse 62 has an amplitude AMP_(A) and apulse width PW_(A). In some examples, each pulse 62 of electricalstimulation signal 60 can have the same amplitude AMP_(A) and pulsewidth PW_(A). In other examples, at least one pulse 62 of signal 60 mayhave a different amplitude AMP_(A) and/or pulse width PW_(A) thananother pulse 62. However, in either example, electrical stimulationsignal 60 has a duty cycle of about 5% to about 50% and a frequency in arange of about 1 Hz to about 1400 Hz or less (e.g., less than or equalto about 1000 Hz). In addition, in some examples, each pulse 62 may havea pulse width PW_(A) in a range of about 0.1 ms to about 5 ms (e.g.,less than or equal to about 1 ms, such as in a range of about 0.5 ms toabout 1 ms).

The duty cycle of electrical stimulation signal 60, which may be theon-time of electrical stimulation signal 60 per unit of time (e.g., onesecond), can be characterized by a product of a frequency and a pulsewidth PW_(A) of pulses 62. For example, for a stimulation signal 60having a frequency of about 800 Hz and a pulse width of about 300microseconds (μs) (0.0003 seconds), stimulation signal 60 may have aduty cycle of about 24%, calculated as follows:

$\begin{matrix}{{{Duty}{Cycle}} = {{\frac{800{pulses}}{1\sec}*\frac{0.0003\sec}{1{pulse}}} = {\frac{0.24\sec{therapy}{``{{on}{}{time}}"}}{1\sec{total}{time}} = {24\%}}}} & \left( {{Equation}1} \right)\end{matrix}$

As another example, for a stimulation signal having a frequency of about300 Hz and a pulse width of about 700 μs, stimulation signal 60 may havea duty cycle of about 21%, calculated as follows:

$\begin{matrix}{{{Duty}{Cycle}} = {{\frac{300{pulses}}{1\sec}*\frac{0.0007\sec}{1{pulse}}} = {\frac{0.21\sec{therapy}{``{{on}{}{time}}"}}{1\sec{total}{time}} = {21\%}}}} & \left( {{Equation}2} \right)\end{matrix}$

In some examples, for a frequency of less than or equal to about 1000Hz, the pulse width PW_(A) of pulses 62 can be selected such thatstimulation signal 60 has a duty cycle of about 20% to about 50%. Inaddition, the amplitude AMP_(A) of stimulation signal 60 can be selectedsuch that the dose of electrical stimulation signal 60 (having thedesired duty cycle) is sufficient to elicit a therapeutic response frompatient 12 when IMD 14 delivers electrical stimulation signal 60 to atarget tissue site in patient 12 (e.g., proximate spinal cord 20, aperipheral nerve, a muscle, or another suitable tissue site, which maybe selected based on the patient condition being treated). For example,in examples in which pulses 62 are substantially similar (e.g.,identical or nearly identical amplitudes AMP_(A) and pulse widthsPW_(A)), the dose of electrical stimulation signal 60 can be determinedto be a product of the amplitude AMP_(A) and pulse width PW_(A) of thepulses 62A-62D, which are the pulses 62 delivered over a 0.01 secondperiod of time. In some examples in which IMD 14 delivers stimulationsignal 60 to patient 12 to spinal cord 20 to treat pain, stimulationsignal 60 may have a duty cycle of about 20% to about 50%, a frequencyin a range of about 1 Hz to about 1400 Hz, and pulses 62 may each have apulse width PW_(A) less in a range of about 0.1 ms to about 5 ms and anamplitude AMP_(A) below a paresthesia threshold of patient 12.

Stimulation generator 34 of IMD 14 may generate and deliver high dutycycle electrical stimulation signal 60 using any suitable technique. Insome examples, stimulation generator 34 may deliver each of the pulses62 with the same electrode combination. In some examples, stimulationgenerator 34 may deliver one or more recharge pulses (also referred toas a “recovery pulse” or a “charge balancing pulse”) after apredetermined number of pulses 62 are delivered, the predeterminednumber being greater than one. Thus, rather than charge balancing on apulse-by-pulse basis (e.g., delivering one recharge pulse after eachpulse 62), in some examples, processor 30 may control stimulationgenerator 34 to deliver one or more recharge pulses after delivery oftwo or more pulses 62. In other examples, processor 30 may controlstimulation generator 34 to deliver pulses to promote charge balance ona pulse-by-pulse basis.

In other examples, stimulation generator 34 may deliver different pulses62 via respective electrode combinations, such that the high pulsedensity electrical stimulation signal is delivered via multiple therapyprograms. For example, under the control of processor 30, stimulationgenerator 34 may deliver pulses 62A, 62C, 62E, 62G with a firstelectrode combination, and deliver pulses 62B, 62D, 62F with a second,different electrode combination. In this example, pulses 62A, 62C, 62E,62G can be part of a first sub-signal delivered via the first electrodecombination, and pulses 62B, 62D, 62F can be part of a second sub-signaldelivered via the second electrode combination. The first and secondsub-signals, when delivered together over time such that the pulses ofthe sub-signals interleaved together as shown in FIG. 4, combine todefine high duty cycle electrical stimulation signal 60. Although twosub-signals are used here as an example, in other examples, stimulationgenerator 34 of IMD 14 may generate and deliver high duty cycleelectrical stimulation signal 60 using any suitable number ofsub-signals. In some examples, stimulation generator 34 may generateeach sub-signal using a respective therapy program, which may be storedas a group in memory 32 of IMD 14 (FIG. 2).

In some examples in which stimulation generator 34 may deliver differentpulses 62 via different electrode combinations, processor 30 may controlstimulation generator 34 may deliver one or more recharge pulses after apredetermined number of pulses 62 are delivered, the predeterminednumber being greater than one. The predetermined number of pulses 62 mayinclude pulses generated according to different therapy programs. Thus,in some examples, stimulation generator 34 may deliver one or morerecharge pulses after pulses of different sub-signals are delivered. Forexample, under the control of processor 30, stimulation generator 34 maydeliver one or more recharge pulses after stimulation generator deliverspulses 62A and 62B, rather than delivering one or more recharge pulsesbetween pulses 62A, 62B, and then again after pulse 62B. In thisexample, stimulation generator 34 may wait to deliver one or morerecharge pulses until after stimulation generator delivers pulses 62Cand 62D, rather than delivering one or more recharge pulses betweenpulses 62C, 62D, and then again after pulse 62B. In other examples,processor 30 may control stimulation generator 34 to deliver rechargepulses to balance charge on a pulse-by-pulse basis.

Stimulation generator 34 can deliver the sub-signals using electrodesfrom a single lead 16A or from two or more leads 16B. For example, underthe control of processor 30, stimulation generator 34 may deliver afirst pulse 62A with electrode 24A of lead 16A together with a housingelectrode of outer housing 34 of IMD 14 and deliver pulse 62B withelectrode 24B of lead 16A together with a housing electrode of outerhousing 34. As another example, under the control of processor 30,stimulation generator 34 may deliver a first pulse 62A with electrodes24A, 24B of lead 16A and deliver pulse 62B with electrodes 24B, 24C ofthe same lead 16A. In another example, stimulation generator 34 maydeliver different pulses 62 with electrodes of different leads.Processor 30 may, for example, control stimulation generator 34 toalternate delivery of pulses 62 between leads 16A, 16B, or controlstimulation generator 34 to otherwise deliver pulses 62 with electrodesof each lead 16A, 16B at different times. For example, under the controlof processor 30, stimulation generator 34 may deliver a first pulse 62Awith electrodes 24A, 24B of lead 16A and deliver pulse 62B withelectrodes 26A, 26B of lead 16B.

Regardless of the number of electrode combinations with whichstimulation generator 34 delivers pulses 62, the combination of pulses62 may combine to define electrical stimulation signal 60 having a dutycycle in a range of about 20% to about 50% and a frequency in a range ofabout 1 Hz to about 1400 Hz.

Delivery of each sub-signal by stimulation generator 34 may generate astimulation field within tissue of the patient, where the stimulationfield may be a volume of tissue through which the electrical currentfrom the delivered sub-signal propagates. The electrode combinationswith which pulses 62 are delivered and the frequency of high duty cycleelectrical stimulation signal 60 can be selected such that thecombination of pulses 62A, 62B (or any other number of pulses 62delivered from any suitable number of different electrode combinations)results in stimulation fields that overlap. The region of overlap of thestimulation fields may be configured to target neural areas responsiveto the high duty cycle mechanisms described herein, e.g., to provide thedesired therapeutic effect. In some examples, the regions of thestimulation fields that do not overlap may not provide any therapeuticeffect.

In some examples, processor 30 controls stimulation generator 34 togenerate and deliver pulses 62 via two or more therapy programs, eachdefining a respective electrode combination. For example, some pulses 62may be part of a first sub-signal defined by a first therapy program anddelivered by stimulation generator 34 via a first electrode combination,and other pulses 62 may be part of a second sub-signal defined by asecond therapy program and delivered by stimulation generator 34 via asecond electrode combination. Stimulation generator 34 may interleavedelivery of pulses of the first and second sub-signals, such that thepulses only partially overlap in time or do not overlap in time.Delivery of the first and second sub-signals may generate respectivestimulation fields within tissue. In some examples, the stimulationfields, individually and when overlapping, have stimulation intensitiesless than at least one of a perception threshold or a paresthesiathreshold of the patient. In addition, in some examples, each pulse ofthe first and second sub-signals has a pulse width less than or equal toabout 5 milliseconds, and stimulation generator 34 may interleavedelivery of pulses of the first and second sub-signals to deliverelectrical stimulation pulses at a frequency in a range of about 1 Hz toabout 1400 Hz. In some examples, processor 30 controls stimulationgenerator 34 to deliver a recharge signal following the delivery of atleast one pulse of each of the first and second electrical sub-signals.

Delivering stimulation signal 60 as multiple sub-signals delivered viarespective electrode combinations may help reduce the charge density atthe electrode-tissue interface of particular electrodes. In addition,delivering stimulation signal 60 via multiple sub-signals may providemore flexibility in programming the electrical stimulation therapy thathas an intensity below the perception or paresthesia threshold intensitylevel of patient 12 because the sub-signals may each have relatively lowstimulation intensities, but due to the overlap in the stimulationfields that may result from the interleaving of the delivery of thesub-signals, the sub-signals may be combined to provide efficaciouselectrical stimulation therapy to patient 12.

In some examples in which stimulation generator 34 generates anddelivers a plurality of sub-signals in order to deliver the electricalstimulation signal having the high duty cycle and frequency less than orequal to about 1400 Hz described herein, stimulation generator 34 mayrecharge at the end of the pulse train, e.g., after the pulses of theplurality of sub-signals are delivered. In other examples, stimulationgenerator 34 may recharge after each delivered pulse.

FIG. 5 is a timing diagram of another example high duty cycle electricalstimulation signal 64 that IMD 14 may generate and deliver to patient12. Electrical stimulation signal 64 includes a plurality of pulses 66.Stimulation signal 64 may include any number of pulses 66, which maydepend on the duration that IMD 14 delivers stimulation signal 64 topatient 12. As with stimulation signal 60 (FIG. 4), stimulation signal64 may have duty cycle of about 5% to about 50%, a frequency in a rangeof about 1 Hz to about 1400 Hz. However, in contrast to stimulationsignal 60, each pulse 66 of stimulation signal 64 has a smaller pulsewidth PW_(B) and a higher amplitude AMP_(B) than each of the pulses 62of stimulation signal 60. The charge density of stimulation signal 64may be similar to (e.g., identical or nearly identical) to stimulationsignal 60, e.g., because the higher amplitude AMP_(B) may compensate forthe decrease in energy delivery resulting from the decrease in pulsewidth PW_(B) relative to pulses 62 of signal 60. As with amplitudeAMP_(A), amplitude AMP_(B) may be less than or equal to a paresthesia orperception threshold of patient 12. In addition, the duty cycle ofsignal 64 can be substantially the same as the duty cycle of signal 60(FIG. 4), despite the smaller pulse width PW_(B), due at least in partto the greater number of pulses 66 per second than signal 60.

Stimulation generator 34 of IMD 14 may generate and deliver high dutycycle electrical stimulation signal 64 using any suitable technique,such as those described with respect to signal 60.

As discussed above, due to potentially different mechanisms of action, apatient may respond differently to the high duty cycle electricalstimulation described herein, which may have a duty cycle of about 5% toabout 50% and a frequency in a range of about 1 Hz to about 1400 Hz,compared to burst electrical stimulation techniques and high frequencyelectrical stimulation techniques.

FIG. 6 is a timing diagram of an example burst electrical stimulationsignal 68, which includes a plurality of pulses 70A-70H (collectively,“pulses 70”). Burst electrical stimulation signal 68 has fewer pulses 70per unit of time (e.g., one second) than high duty cycle electricalstimulation signals 60, 64 (FIGS. 4 and 5). During a particular periodof time, e.g., one second as shown in FIG. 6, an IMD delivers a burst ofpulses 70A-70D of electrical stimulation signal 68 for a first timeperiod 72, which is immediately followed by a second period of time 74during which the IMD does not deliver any electrical stimulation, but,rather, delivers one or more recovery pulses. Second time period 74 maybe referred to as a “recovery period.” After second time period 74, theIMD 14 may deliver another burst of pulses 70E-70H, which may befollowed by another recovery period. First and second time periods 72,74 may be substantially equal (e.g., equal or nearly equal) in someexamples, and different in other examples.

In contrast to burst electrical stimulation signal 68, delivery of highduty cycle electrical stimulation signals 60, 64 by IMD 14 may providebetter targeting of target tissue sites. For a given dose, burstelectrical stimulation signal 68 may result in activation of more neuraltissue (e.g., a larger volume of tissue) than high duty cycle electricalstimulation signals 60, 64, which may each provide electricalstimulation with a higher duty cycle than burst electrical stimulationsignal 68 and with smaller pulse widths.

FIG. 7 is a timing diagram of an example high frequency electricalstimulation signal 76, which includes a plurality of pulses 78. Highfrequency electrical stimulation signal 76 has a higher frequency thanhigh duty cycle electrical stimulation signals 60, 64 (FIGS. 4 and 5,such that signal 76 has a greater number of pulses 78 per unit of timethan high duty cycle electrical stimulation signals 60, 64). Forexample, high frequency electrical stimulation signal 76 may have afrequency of 1500 Hz to about 100 kiloHz, or greater, whereas high dutycycle electrical stimulation signals 60, 64 may each have a frequencyless than or equal to about 1400 Hz.

For a given duty cycle, high frequency electrical stimulation signal 76may result in activation of more neural tissue than high duty cycleelectrical stimulation signals 60, 64, which have pulses 62, 66,respectively, with higher pulse widths than pulses 78 of high frequencyelectrical stimulation signal 76. The lower frequency of high duty cycleelectrical stimulation signals 60, 64 may allow for a larger therapeuticwindow for the pulse amplitudes AMP_(A) and AMP_(A), which may help aclinician tailor the electrical stimulation to a particular patient toallow for different neural mechanisms to be activated in order to elicita therapeutic response from the patient. The therapeutic window for thepulse amplitudes AMP_(A) and AMP_(A) can be, for example, the range ofamplitude values that provide efficacious therapy to patient 12 withoutresulting in undesired side effects.

The electrical stimulation parameter values with which IMD 14 maygenerate and deliver the high density electrical stimulation describedherein, having a duty cycle of about 5% and about 50%, a frequency lessin a range of about 1 Hz to about 1400 Hz (e.g., less than or equal toabout 1000 Hz), and a pulse width of about 5 ms or less, may be selectedusing any suitable technique. FIG. 8 is a flow diagram of an exampletechnique for selecting the electrical stimulation parameter values.While FIG. 8 is described with respect to processor 30 of IMD 14, inother examples, processor 50 of programmer 18 may perform any part ofthe technique described with respect to FIG. 8, alone or in combinationwith processor 30 of IMD 14.

In the technique shown in FIG. 8, processor 30 determines a paresthesiaor perception threshold intensity level for patient 12 (80), e.g., usingthe technique described below with respect to FIG. 9, by retrieving astored paresthesia or perception threshold intensity level from memory32 (FIG. 2), or by receiving a paresthesia or perception thresholdintensity level from another device, e.g., programmer 18. Processor 30may, for example, determine the paresthesia threshold (80), determinethe perception threshold (80), determine the lower of the paresthesiathreshold intensity level or the perception threshold intensity levelfor patient 12 (80), or determine the higher of the paresthesiathreshold intensity level or the perception threshold intensity levelfor patient 12.

A paresthesia threshold intensity level may be a lowest determinedelectrical stimulation intensity level at which patient 12 firstperceives paresthesia from the electrical stimulation delivered by IMD14. A perception threshold intensity level may be a lowest determinedelectrical stimulation intensity level at which patient 12 firstperceives the electrical stimulation delivered by IMD 14. In some cases,depending on the patient and/or the target electrical stimulation sitewithin the patient, the patient may first perceive the electricalstimulation delivered by IMD 14 as paresthesia. Thus, in some cases, theperception threshold intensity level may be substantially the same(e.g., identical or nearly identical) as the paresthesia thresholdintensity level. In other cases, however, a patient may first perceivethe electrical stimulation as a sensation different than paresthesia.Thus, some cases, the perception threshold intensity level may bedifferent than the paresthesia threshold intensity level. In theseexamples, a clinician may program IMD 14 and/or programmer 18 to useeither the perception or paresthesia threshold intensity levels toselect the electrical stimulation parameter with the technique shown inFIG. 8.

After determining one or both of the paresthesia threshold intensitylevel or the perception threshold intensity level, processor 30 maydetermine a strength-duration curve based on the determined one or bothof the paresthesia or perception threshold intensity level and one ormore selected electrical stimulation signal frequencies (82). Astrength-duration curve may describe the relationship between a strengthof electrical stimulation and duration, e.g., for a particularphysiological response, such as a response below the paresthesia orperception threshold of patient 12. The strength of electricalstimulation may be a function of, for example, any one or more of thevoltage or current amplitude value of the stimulation signal, frequencyof stimulation signals, signal duration (e.g., pulse width in the caseof stimulation pulses), duty cycle, and the like.

An example of a strength duration curve is an amplitude-pulse widthcurve. The amplitude-pulse width curve may reflect, for a selectedstimulation frequency, different combinations of amplitude and pulsewidth values that contribute to a stimulation field in a substantiallysimilar manner. For example, the amplitude-pulse width curve mayindicate that a first electrical stimulation signal with a firstamplitude and a first pulse width, and a second electrical stimulationsignal having a higher amplitude pulse with a shorter pulse width (i.e.,shorter than the first pulse width) may both provide electricalstimulation therapy below the paresthesia or perception threshold ofpatient 12. Each position on the amplitude-pulse width curve, or eachposition within a particular range of positions along theamplitude-pulse width curve, may result in a substantially similarstimulation energy when the other therapy parameter values, such as afrequency, remain substantially constant (e.g., the other therapyparameter values may remain within a particular range of therapyparameter values, such as within a 10% window or less from the valuesdefined by the therapy program). Thus, for a given stimulationfrequency, the amplitude-pulse width curve may define, e.g., via theamplitude-pulse width combinations associated with the area under thecurve and/or along the curve, the amplitude and pulse width combinationsthat provide electrical stimulation therapy having an intensity levelbelow the paresthesia or perception threshold intensity level of patient12.

For a given frequency (e.g., in a range of about 1 Hz to about 1000 Hz),based on the strength-duration curve, processor 30 may determine thepulse width and amplitude combination that provides efficaciouselectrical stimulation therapy to patient 12 and also has a stimulationintensity below the paresthesia or perception threshold of patient 12(84). Processor 30 may, automatically or in response to user inputprovided via programmer 18, control stimulation generator 34 to generateand deliver electrical stimulation therapy to patient 12 with thefrequency associated with the strength-duration curve, a selectedcombination of electrodes 24, 26, and a plurality of pulse width andamplitude combinations along the strength-duration curve or below theamplitude-pulse width curve. Processor 30 may determine whether any ofthe selected pulse width and amplitude combinations provides efficaciouselectrical stimulation therapy for patient 12, e.g., based on patient 12input or input from another entity received via programmer 18, based oninput from a sensing module of IMD 14 or a separate sensing module, orany combination thereof. Processor 30 may generate one or more therapyprograms based on the one or more pulse width and amplitude combinationsthat provide efficacious electrical stimulation therapy to patient 12,together with the selected frequency and electrode combination (86).

In some examples in which stimulation generator 34 generates anddelivers the high duty cycle electrical stimulation therapy via aplurality of sub-signals delivered via respective electrodecombinations, processor 30 may determine a strength-duration curve foreach electrode combination. Thus, for each electrode combination, therespective strength-duration curve may indicate a plurality ofcombinations of electrical stimulation parameters (e.g., amplitude andpulse width for a given frequency) that provide a charge per pulse belowthe paresthesia or perception threshold of patient 12. Based on thestrength-duration curves, processor 30, alone or based on input from aclinician, may determine, for each of the electrode combinations, one ormore therapy programs that provide a relatively high charge per pulse(e.g., the relatively highest charge per pulse that remains at or belowthe paresthesia or perception threshold of patient 12). Each therapyprogram may define a sub-signal. Processor 30, alone or based on inputfrom a clinician, may then determine a frequency to interleave the twoor more sub-signals.

In some examples, to determine the therapy programs, processor 30 maydetermine one or more test therapy programs that define relatively widepulse widths and relatively low frequencies of the sub-signals, controlstimulation generator 34 to generate and deliver electrical stimulationto patient 12 according to the test therapy programs, and, if thedelivered electrical stimulation therapy is not sufficientlyefficacious, processor 30 may modify one or more of the test therapyprograms until the electrical stimulation provides efficaciousstimulation therapy for patient 12. The efficacy of the electricalstimulation therapy can be based on input from patient 12, from one ormore sensed physiological parameters, or any combination thereof.Processor 30 may modify one or more of the test therapy programs by, forexample, incrementally narrowing the pulse width (e.g., by apredetermined increment) and/or incrementally increasing the frequency(e.g., by a predetermined increment).

Processor 30 may store the one or more therapy programs 40 in memory 32of IMD 12 or a memory of another device for later delivery of electricalstimulation therapy to patient 12 (86). Processor 30 may controlstimulation generator 34 to generate and deliver electrical stimulationtherapy to patient 12 in accordance with the one or more therapyprograms 40.

In some cases, therapeutic efficacy of electrical stimulation therapydelivered by IMD 14 may change as the patient posture state (e.g., aparticular patient posture or a combination of posture and activity)changes. Efficacy may refer to a combination of complete or partialalleviation of symptoms alone, or in combination with no side effects oran acceptable or tolerable degree of undesirable side effects. In someexamples, processor 30 of IMD 14 may be configured to adjust one or moretherapy parameter values based on different postures and/or activitiesengaged by patient 12 to maintain effective therapy, e.g., by selectingselect different therapy programs based on a posture state of patient12. In these examples, processor 30 may determine the paresthesia orperception threshold of patient 12 for each of a plurality of differentposture states and determine one or more therapy programs 40 for each ofthe posture states using the technique shown in FIG. 8 based on therespective paresthesia or perception threshold.

FIG. 9 is a flow diagram of an example technique by which processor 30of IMD 14 can determine at least one of the perception or paresthesiathreshold intensity level for patient 12. In some examples, processor 30is configured to determine the perception threshold intensity level,while in other examples, processor 30 is configured to determine theparesthesia threshold intensity level or both the perception andparesthesia threshold intensity level.

The perception or paresthesia threshold intensity level can bepatient-specific, as well as specific to a target tissue site withinpatient 12. Thus, a perception or paresthesia threshold intensity levelcan be determined for each target tissue site to which IMD 14 deliversstimulation therapy. In some examples, processor 30 of programmer 18 mayimplement the technique illustrated in FIG. 9 automatically, e.g.,without user intervention or control after initiating the technique. Inother examples, processor 30 may implement the technique illustrated inFIG. 9 under control of a user, such as a clinician, who controlsprocessor 30 via programmer 18. While FIG. 9 is described with respectto processor 30 of IMD 14, in other examples, processor 50 of programmer18 may perform any part of the technique described with respect to FIG.9, alone or in combination with processor 30 of IMD 14.

In accordance with the technique shown in FIG. 9, processor 30 setsstimulation parameter values such that the stimulation parameter valuesdefine a relatively low stimulation intensity, e.g., an intensity belowan expected perception or paresthesia threshold intensity (90). Theinitial stimulation parameter values may be selected by a clinician insome examples. In some examples in which processor 30 controlsstimulation generator 34 to generate and deliver stimulation to patient12 in the form of electrical pulses, the stimulation parameters includeat least one of a voltage or current amplitude, a pulse width, a pulserate, or a duty cycle. In examples in which processor 30 controlsstimulation generator 34 to deliver stimulation to patient 12 in theform of a continuous waveform, the stimulation parameters include atleast one of a voltage amplitude, a current amplitude, a frequency, awaveform shape, or a duty cycle.

In either case, processor 30 sets the stimulation parameters torespective values to define a stimulation intensity, and controlsstimulation generator 34 to deliver stimulation to patient 12 at the setstimulation intensity (defined by the selected stimulation parametervalues) (92). During therapy delivery or after stimulation generator 34delivers stimulation to patient 12, processor 30 determines whetherpatient 12, a clinician, or patient caretaker has provided inputindicating patient 12 has perceived the electrical stimulation orindicating paresthesia resulted from the electrical stimulation (94).Patient 12, the clinician, or patient caretaker can provide the input,e.g., via user interface 54 of programmer 18 or directly via IMD 14. Forexample, a motion sensor can be integrated into or on a housing of IMD14, and the motion sensor can be configured to generate a signal that isindicative of patient 12 tapping IMD 14 through the skin. The number,rate, or pattern of taps may be associated with the input indicative ofstimulation perception or paresthesia, and processor 30 may identify thetapping by patient 12 to determine when patient input is received. Whenthe input is received via user interface 54 of programmer 18, processor50 of programmer 18 may transmit a signal indicative of the input to IMD14 via the respective telemetry modules 56, 36.

When processor 30 has not received an indication of the input indicativeof the stimulation perception or paresthesia within a predetermined timeperiod during or immediately after delivery of the stimulation accordingto the selected stimulation intensity (“NO” branch of block 94),processor 30 again sets the stimulation intensity, e.g., by adjusting atleast one stimulation parameter value to increase a stimulationintensity of the stimulation signal (90). For example, processor 30 mayincrease a voltage amplitude or a current amplitude to increase thestimulation intensity. In some examples, processor 30 changes a value ofonly one of the stimulation parameters while the remaining parametersare kept approximately constant. The stimulation parameter that isselected may be known to affect stimulation intensity. In otherexamples, processor 30 may adjust a combination of two or morestimulation parameters to increase stimulation intensity.

After modifying the one or more stimulation parameter values, processor30 controls stimulation generator 34 to deliver stimulation to patient12 using the newly defined stimulation parameter values (92). In thisway, processor 30 can implement an iterative procedure to determine theperception or paresthesia threshold intensity for patient 12, and, insome examples, for a specific target tissue site within patient 12.

In response to not receiving input indicative of patient perception orparesthesia is received within a predetermined time period during orimmediately after delivery of the stimulation according to the selectedstimulation intensity (“NO” branch of block 94), processor 30 may againadjust at least one stimulation parameter value to increase astimulation intensity of the stimulation signal (90). This process mayrepeat until processor 30 receives input indicative of patientperception or paresthesia within a predetermined time period during orimmediately after delivery of the stimulation according to the selectedstimulation intensity. In response to receiving the input (“YES” branchof block 94), processor 30 may store the stimulation intensity level asthe patient perception threshold intensity level and/or paresthesiathreshold intensity level (depending on the whether the responseindicates patient perception of the electrical stimulation or resultingparesthesia, respectively) in memory 32 of IMD 14 (FIG. 2) or in anothermemory (e.g., memory 52 of programmer 18) (96).

In addition, processor 30 may define stimulation parameter values forthe therapy programs 40 (FIG. 2) for providing the high duty cycleelectrical stimulation techniques described herein based on thedetermined patient perception threshold intensity level and/orparesthesia threshold intensity level, e.g., using the techniquedescribed with respect to FIG. 8. For example, processor may definestimulation parameter values for the therapy programs 40 (FIG. 2) thatresult in a stimulation intensity level less than or equal to one orboth of the patient perception threshold intensity level or paresthesiathreshold intensity level.

While the techniques described above are primarily described as beingperformed by processor 30 of IMD 14 or processor 50 of programmer 18, inother examples, one or more other processors may perform any part of thetechniques described herein alone or in addition to processor 30 orprocessor 50. Thus, reference to “a processor” may refer to “one or moreprocessors.” Likewise, “one or more processors” may refer to a singleprocessor or multiple processors in different examples.

The techniques described in this disclosure, including those attributedto IMD 14, programmer 18, or various constituent components, may beimplemented, at least in part, in hardware, software, firmware or anycombination thereof. For example, various aspects of the techniques maybe implemented within one or more processors, including one or moremicroprocessors, DSPs, ASICs, FPGAs, or any other equivalent integratedor discrete logic circuitry, as well as any combinations of suchcomponents, embodied in programmers, such as clinician or patientprogrammers, medical devices, or other devices.

In one or more examples, the functions described in this disclosure maybe implemented in hardware, software, firmware, or any combinationthereof. If implemented in software, the functions may be stored on, asone or more instructions or code, a computer-readable medium andexecuted by a hardware-based processing unit. Computer-readable mediamay include computer-readable storage media forming a tangible,non-transitory medium. Instructions may be executed by one or moreprocessors, such as one or more DSPs, ASICs, FPGAs, general purposemicroprocessors, or other equivalent integrated or discrete logiccircuitry. Accordingly, the term “processor,” as used herein may referto one or more of any of the foregoing structure or any other structuresuitable for implementation of the techniques described herein.

In addition, in some aspects, the functionality described herein may beprovided within dedicated hardware and/or software modules. Depiction ofdifferent features as modules or units is intended to highlightdifferent functional aspects and does not necessarily imply that suchmodules or units must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modules orunits may be performed by separate hardware or software components, orintegrated within common or separate hardware or software components.Also, the techniques could be fully implemented in one or more circuitsor logic elements. The techniques of this disclosure may be implementedin a wide variety of devices or apparatuses, including an IMD, anexternal programmer, a combination of an IMD and external programmer, anintegrated circuit (IC) or a set of ICs, and/or discrete electricalcircuitry, residing in an IMD and/or external programmer.

Various aspects of the disclosure have been described. These and otheraspects are within the scope of the following claims.

What is claimed is:
 1. A method comprising: determining a paresthesia orperception threshold for a patient; determining, for a selectedfrequency of an electrical stimulation signal, a strength-duration curvebased on the paresthesia or perception threshold; and determining, byprocessing circuitry and based on the strength-duration curve, a set ofone or more electrical stimulation parameter values for generating theelectrical stimulation signal having stimulation intensity less than atleast one of the perception threshold or the paresthesia threshold ofthe patient, and having a duty cycle in a range of about 5% to about50%, a frequency in a range of about 1 Hertz to about 1400 Hertz, and apulse width in a range of about 0.1 millisecond to about 5 milliseconds.2. The method of claim 1, wherein the frequency is in a range of about 1Hertz to about 1000 Hertz.
 3. The method of claim 1, wherein the pulsewidth is in a range of about 0.1 millisecond to about 1 millisecond. 4.The method of claim 1, wherein the duty cycle is in a range of about 10%to about 40%.
 5. The method of claim 1, wherein the duty cycle is anon-time of the pulses per second.
 6. The method of claim 1, whereindetermining the set of one or more electrical stimulation parametervalues comprises determining a plurality of therapy programs eachincluding a respective electrode combination, the method furthercomprising delivering the electrical stimulation signal to the patientby at least interleaving delivery of electrical stimulation according tothe plurality of therapy programs.
 7. The method of claim 1, wherein thepulses have substantially the same pulse width.
 8. The method of claim1, wherein at least two of the pulses have different pulse widths. 9.The method of claim 1, wherein the strength-duration curve defines arelationship between amplitude and pulse width for the selectedfrequency.
 10. The method of claim 1, further comprising delivering theelectrical stimulation signal according to the set of one or moreelectrical stimulation parameter values.
 11. A system comprising:processing circuitry configured to: determine a paresthesia orperception threshold stimulation intensity level of a patient,determine, for a selected frequency, a strength-duration curve based onthe paresthesia or perception threshold stimulation intensity level, anddetermine, based on the strength-duration curve, a set of one or moreelectrical stimulation parameter values for generating an electricalstimulation signal having stimulation intensity less than at least oneof the perception threshold or the paresthesia threshold of the patient,and having a duty cycle in a range of about 5% to about 50%, a frequencyin a range of about 1 Hertz to about 1400 Hertz, and a pulse width in arange of about 0.1 millisecond to about 5 milliseconds.
 12. The systemof claim 11, wherein the frequency is in a range of about 1 Hertz toabout 1000 Hertz.
 13. The system of claim 11, wherein the pulse width isin a range of about 0.1 millisecond to about 1 millisecond.
 14. Thesystem of claim 11, wherein the duty cycle is in a range of about 10% toabout 40%.
 15. The system of claim 11, wherein the processing circuitryis configured to determine the set of one or more electrical stimulationparameter values by at least determining a plurality of therapy programseach including a respective electrode combination, the method furthercomprising delivering the electrical stimulation signal to the patientby at least interleaving delivery of electrical stimulation according tothe plurality of therapy programs.
 16. The system of claim 11, whereinthe pulses have substantially the same pulse width.
 17. The system ofclaim 11, wherein at least two of the pulses have different pulsewidths.
 18. The system of claim 11, wherein the strength-duration curvedefines a relationship between amplitude and pulse width for theselected frequency.
 19. The system of claim 11, further comprising astimulation generator, wherein the processing circuitry is configured tocontrol the stimulation generator to deliver the electrical stimulationsignal according to the set of one or more electrical stimulationparameter values.
 20. A non-transitory computer readable mediumcomprising instructions that, when executed by processing circuitry,cause the processing circuitry to: determine a paresthesia or perceptionthreshold stimulation intensity level of a patient, determine, for aselected frequency, a strength-duration curve based on the paresthesiaor perception threshold stimulation intensity level, and determine,based on the strength-duration curve, a set of one or more electricalstimulation parameter values for generating an electrical stimulationsignal having stimulation intensity less than at least one of theperception threshold or the paresthesia threshold of the patient, andhaving a duty cycle in a range of about 5% to about 50%, a frequency ina range of about 1 Hertz to about 1400 Hertz, and a pulse width in arange of about 0.1 millisecond to about 5 milliseconds.